Seismic Functional Fragility Analysis Of High-Speed Railway Long-Span Extradosed Cable-Stayed Bridge

High-speed railways are of great significance in the implementation of the natioanl major strategies of the"the Belt and Road"and"the 14th Five-Year Plan Period".As a lifeline engineering,the damage and destruction of bridges under earthquake not only affect the structural safety,but also has a great impact on the operation of railway trunk line,emergency rescue and running safety.Extradosed cable-stayed bridges have been widely used in high-speed railway because of its unique advantages.However few attempts of the seismic damage patterns and damage laws of high-speed railway bridges are devoted to in civil engineering field.Especially when considering the high requirements of high-speed railway running safety,there is still a lack of systematic research on the following issues,such as the definition of functional damage and the selection of damage index under earthquake,bridges modal identification,the selection of ground motion intensity parameters,the similarities and differences between functional damage and bearing safety damage,and the bridge’s damage exceedance probabilities.Therefore,in this thesis,taking a high-speed railway long span extradosed cable-stayed bridge as a case-study and on the basis of existing research,literature research,theoretical derivation,numerical simulation,probability analysis and existing test reference demonstration are adopted to carry out the research.And then,the related knowledge of bridge engineering,earthquake engineering,finite element theory,structural reliability,probability and mathematical statistics are comprehensive used to solve the above problems.The following results are obtained:（1）Based on the running safety of high-speed railway bridges,the conception of"functional sesimic fragility"is proposed in this thesis,which is an extension of the traditional bridge seismic bearing fragility research,and it is also a further embodiment of multi-level performance seismic design.It is clear that there are differences and connections between“functional state”and“bearing state”damage of the bridges.（2）The"functional state"and"bearing state"damage limit states are determined,and the main beam angle,the displacement at top of pier,curvature of pier column and tower,support displacement and cable force are selected as damage indexes.And then the index evaluation standard and quantitative method applicable to each bridge damage form are established.（3）The"average modal strain energy ratio"is proposed as an index for the identification of the dominant mode of extradosed cable-stayed bridges,and the threshhold is set to 0.01 to identify the dominant mode.The dominant modes in x,y and z directions of the bridge are identified respectively,and the dominant modes are verified through time history analysis and mode decomposition response spectrum method to verify the correctness of the theoretical derivation.The identification of the dominant mode of bridge can provide scientific guarantee for clarifying the dynamic characteristics and seismic damage analysis.（4）A calculation method for optimizing ground motion intensity parameters is proposed to obtain the parameter S_a,gui that can fully couple the natural vibration characteristics of bridge and the seismic spectrum information.Combined with the analysis of"functional state"and"bearing state"fragility engineering demand parameters,it shows that the parameter S_a,gu has obvious advantages by demonstrating with traditonal ground motion intensity parameters.（5）The influence of structural uncertainty on bridge dynamic response is explored,and the sensitivity of various engineering demand parameters to random parameters such as materials,structural characteristics and structural geometric dimensions is analyzed.The primary,secondary and insensitive parameters affecting bridge dynamic response are obtained,which further verifies the rationality and applicability of the established damage indexes.It provides a strong guarantee for the seismic fragility research of high-speed extradosed cable-satyed bridges.（6）Research on seismic fragility of extradosed cable-stayed bridge shows that the uncertainty have great influences on both probabilistic seismic demand analysis and damage exceedance probabilities.The damage rules and damage exceedance probabilities of different members under different limit states are obtained respectively when suffering longitudinal and transverse earthquakes.The order of component damage is in accordance with the results of the same type of bridge shaking table test in China,which proves the accuracy of the research in this thesis.At the same time,the relationship between"functional damage"and"bearing damage"is also explored,which further reflects the importance of functional safety in high-speed railway bridges.（7）The comparison of bridge system function and bearing damage further shows that with the development of damage degree,the function damage of high-speed railway extradosed cable-stayed bridge will become a prominent problem,which needs to be focused on in seismic design,damage analysis and evaluation.The research on"functional damage"of bridge proposed in this thesis has theoretical and practical significance,and can be used in the seismic fragiliy research of high-speed railway long-span bridges.